The present invention relates to a color electrostatic copying machine. In one type of prior art color copying machine a photoconductive drum is radiated with a light image of an original document three times through primary color filters respectively. After each imaging operation a toner substance of the complementary primary color is applied to the drum to produce a toner image which is transferred to a copy sheet. The three toner images superimposed on each other on the copy sheet form a full color copy of the original document and are thermally or otherwise fixed to the copy sheet. In a variation of this process, electrostatic images are formed on the drum, transferred to the copy sheet, and the toner substances are applied to the copy sheet for development. Although satisfactory copies may be produced by such a process, the fact that the drum must be imaged and developed three times to produce one copy makes the process impractical for high speed color copying.
To increase the copying speed, an electrostatic copying machine has been developed which comprises three separate optical imaging systems and three photoconductive drums which are used for the three primary colors respectively. The three drums are imaged in one scan or pass of an original document and the toner images are successively transferred to a copy sheet carried into engagement with the drums by an endless belt.
Two problems have remained heretofore unsolved in such a high speed color copying machine. The first arises from the fact that a transfer charge of a polarity opposite to an electrostatic charge on the toner substances must be applied to the copy sheet to effect transfer of the toner images thereto. Although the first toner image is transferred effectively, the charge of the toner substance constituting the first toner image interferes with the transfer of the second toner image. The third transfer operation is even worse since the charge of the first two toner images interferes with the transfer of the third toner image. This problem is compounded by the discharge effect of the belt which occurs when the belt and copy sheet move out of engagement with the drums.
Another problem is that of maintaining the copy sheet in tight engagement with the belt throughout the three transfer operations. Slippage of the copy sheet on the belt will result in the toner image transfers being out of register.
The fact that the transfer efficiency progressively decreases for the second and third transfer operations adversely affects the color purity and resolution. The best copies are produced when the colored toner images are overlayed on each other on the copy sheet and thermally fixed in place. However, the reduction in transfer efficiency decreases the electrostatic adhesive force of the toner images to the copy sheet and allows the toner images to mix together or smear. These factors have heretofore been major deterents to the widespread commercial applicability and acceptance of this type of high speed color electrostatic copying machine.